Electronic components, in particular solid state electrolytic capacitors in which electrically conductive macromolecules are used, are described as one example of the conventional art. In recent years, there has been a noticeable increase in the frequency and the current of integrated circuits of electronic devices in which solid state electrolytic capacitors are used. Accordingly, there is a demand for solid state electrolytic capacitors whose equivalent series resistance (abbreviated as “ESR”) is low, that have a large capacity and that have small losses. The conventional method for manufacturing an internal electrode (that is to say, a capacitor element) of a solid state electrolytic capacitor is illustrated for solid state electrolytic capacitors. First of all, valve metal (for example tantalum metal) that is to be the anodic conductor is anodized in an electrolytic solution such as phosphoric acid to form an oxide film layer (dielectric layer) on the surface. Next, a solid state electrolyte is formed on the surface of the oxide film layer. Manganese dioxide, which can be formed by, for example, immersing the anodic conductor in a manganese nitrate solution, withdrawing, and then firing, is known as a solid state electrolyte. Finally, a cathodic conductor is formed on the solid state electrolyte. A laminated body of, for example, a carbon layer and a silver-surface conductive resin layer can be used for the cathodic conductor. In order for the capacitor element to connect electrically with the exterior, an anodic lead terminal and a cathodic lead terminal respectively are connected to the anodic conductor and the cathodic conductor.
Although the above-noted members individually may influence the ESR by their own resistance, the solid state electrolyte is the material that should be given the most consideration with respect to resistance. In order to reduce the resistance of the solid state electrolyte, it has been proposed that an electrically conductive macromolecular material whose conductivity is higher than that of manganese dioxide (which has a conductivity of about 0.1 S/cm), and this has come into regular use. For example, if polypyrrole is used, then it is possible to realize a conductivity of about 100 S/cm. In addition to pyrrole, compounds such as aniline, thiophene and 3,4-ethylenedioxythiophene are known as monomers for constituting electrically conductive macromolecular material. Methods for forming electrically conductive macromolecular layers are divided broadly into chemical oxidation polymerization and electrolytic oxidation polymerization.
Contact resistance between layers also affects ESR. In Patent Reference 1, below, which is by the applicant of the present application, it has been disclosed that by mixing electrically conductive polymer micro-particles into the electrically conductive macromolecular layer, contact resistance between the electrically conductive macromolecular layer and the cathodic conductor is reduced by the surface roughness that is formed by the micro-particles. In the method described in this publication, the electrically conductive macromolecular layer is formed by a chemical oxidation polymerization method in which a polymerization solution that is a dispersion of the electrically conductive polymer micro-particles is used.
In order to greatly increase the capacitance of capacitors, it also has been proposed to form the electrically conductive macromolecular layer in particle form. In Patent Reference 2 below, it has been disclosed that particulate polypyrrole having a particle diameter of 0.2 μm or less is formed by chemical oxidation using a polymerization solution in which the molar ratio of the mixture of oxidizing agent to monomer is at least 1. If the particle diameter of the electrically conductive macromolecular layer is reduced, then delamination of the layer can be suppressed and it is easier to gain use of dormant capacitance contained within the dielectric layer.
In Patent Reference 3 below, a method for manufacturing a solid state electrolytic capacitor is disclosed, the method including a process for forming an electrically conductive polymer layer on a synthetic film by immersing a capacitor element in a solution that includes a monomer, which becomes the electrically conductive polymer by oxidation polymerization, and an oxidizing agent, after which the capacitor element is left to stand in air at a temperature of about 30° C. to 50° C. and a relative humidity of at least about 60%. This is with the object of clarifying preferable conditions for forming an electrically conductive polymer layer by chemical polymerization on a capacitor element that includes an anodic member on which a synthetic film is formed, and providing a solid state electrolytic capacitor that is compact, that has a large capacity, that has a low ESR and that has superior productivity.
Patent Reference 1: JP 2000-232036A
Patent Reference 2: JP H8-45790A
Patent Reference 3: JP H10-64761A
As given above, numerous investigations have been carried out with regard to solid state electrolytic capacitors in which electrically conductive macromolecular layers are used as solid state electrolytes. However, compatibility between low ESR and high capacity in solid state electrolytic capacitors, and also realization of low losses and reduction of leakage current have not yet been sufficiently achieved.